A chemical study of the sapogenin content has been conducted on leek, Allium porrum L., sown and collected at different times. As a result of this investigation, a variability in the sapogenin composition depending on the climate was observed. In addition, a novel sapogenin named porrigenin C (1a) and its 25S epimer (1b) have been isolated, together with the previously described sapogenins 2-5 and the known compounds agigenin, diosgenin, beta-chlorogenin, and 24-ethylcholesta-(6-acyl)- 3-O-beta-D-glucoside. The chemical structure of the novel compound was elucidated by a detailed spectroscopic analysis based mainly on 1D and 2D NMR techniques. Porrigenin C showed a considerable antiproliferative activity on four tumor cell lines in vitro.